html, body {
    height: 100%;
    margin: 0;
    background-color: #eee;
}

body {
    background-image: url("../img/ab.jpg");
    background-repeat: no-repeat;
    background-size: 100%;
    background-position: 50% 50%;
    cursor: pointer;
}
.overlay-a {
    position: absolute;
    left: 0;
    top: 0;
    bottom: 0;
    right: 38%;
    background: none;
    display: none;
    cursor: pointer;
    z-index: 1;
}

.overlay-a.active {
    background-image: url("../img/a2.png");
    background-repeat: no-repeat;
    background-size: 100%;
    background-position: 50% 50%;
}

.overlay-b {
    position: absolute;
    left: 37%;
    top: 0;
    bottom: 0;
    right: 0;
    background: none;
    display: none;
    cursor: pointer;        
    z-index: 1;
}

.overlay-b.active {
    background-image: url("../img/b2.png");
    background-repeat: no-repeat;
    background-size: 100%;
    background-position: 50% 50%;
}